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declare @salesstaff varchar(50)
set @salesstaff = null
SELECT @SalesStaff = SalesStaff FROM bdCustomerAllocate WHERE Departmentid = 4 AND CustomerID = -111
if ( @salesstaff  is   null )
begin
    print 'null'
end
else
begin
    print 'sdf'
end

若数据库中没有此记录，则不会给 @SalesStaff赋值，它会保留原值，故最好在赋值前将其设为NULL